This invention relates to an optical amplifier for an optical communications system, and in particular a broadband optical amplifier, for example for use within WDM (wavelength division multiplexing) systems.
To support the ever increasing demand for bandwidth in WDM systems, broadband optical amplifiers are required for use in such optical communications applications. Erbium Doped Fiber Amplifiers (EDFAs) are commonly used in these applications. Although EDFAs can have excellent characteristics over certain wavelength bands, the available spectral band width is limited. The amplification band of an EDFA results from the radiative decay of excited electrons within the Er atoms and since not all energy level transitions are equally likely, input light at a wavelength corresponding to the most likely energy level transition experiences a higher gain than input light at other wavelengths. This creates a non-flat gain profile of the amplifier, known as gain ripple, which is a well known problem.
One proposed way of obtaining optical amplification over a broad bandwidth is achieved by providing amplification in two discreet wavelength bands, the Conventional (C) band of approximately 1528 nm to 1563 nm and the Long (L) band of approximately 1570 nm to 1605 nm. The amplifiers for the two bands are configured differently, for example using different lengths of doped fiber. This requires input signals to the amplifier to be split into the two bands, amplified separately and then recombined at an output of the amplifier. This is difficult since wavelength-dependent demultiplexing hardware is required to split the input signals, two separate amplifiers are needed for the respective demulitplexed bands (C and L) and then multiplexing hardware is needed to recombine the separately amplified signals. This configuration is therefore expensive and introduces a noise penalty to the amplified signal and also introduces additional loss which must be compensated for by extra gain in the amplifier.
It is not possible at present to use a single Er doped fiber as a broadband optical amplifier as the gain ripple problem described above is exacerbated as the bandwidth increases. Over a bandwidth of, say, 1530 nm to 1610 nm a gain ripple of over 30 dB is common. The ripple can be removed with a discrete filter such as a single filter positioned at the amplifier output, but this wastes power thereby reducing the efficiency of the amplifier and making the amplifier impractical.
A further problem with such amplifiers is the propagation and amplification of spontaneous emission noise leading to Amplified Spontaneous Emission (ASE). Spontaneous emission occurs throughout the length of the Er fiber. Thus, spontaneous emission at either end of the amplifier experiences the full gain of the amplifier in either the forwards or backwards direction. Once this ASE reaches the opposite end of the amplifier its power can be significant. The backwards propagating ASE has the largest effect as the signal power at the input of the amplifier is small and is thus easily dominated by the ASE as opposed to the signal thereby reducing the amplifier efficiency and causing poor noise performance. Similarly, if any one channel power is significantly larger than the others, the same problems occur.
According to the present invention, there is provided an optical amplifier comprising an Erbium doped optical fiber and a plurality of optical filtration elements provided at respective positions along the fiber, wherein the wavelength response of each of the optical filtration elements is selected to correspond to a parameter of the optical fiber at the respective position along the fiber, thereby flattening the gain of the amplifier with respect to wavelength over a predetermined bandwidth.
The invention provides an EDFA having a plurality of optical filtration elements provided at respective positions along the fiber having wavelength responses tuned, for example, to the gain profile of the Er doped fiber at that point. Each optical filtration element can then be tuned to the wavelength which corresponds to a peak in the gain profile of the amplifier at that point, which would ordinarily provide unacceptable level of gain of the ASE. Thus, over the entire length of the Er fiber the gain experienced by a propagating signal can be maintained substantially flat with respect to wavelength.
Preferably, the optical filtration elements are non reflective gratings formed in the erbium doped fiber.
Alternatively, the optical filtration elements are optical filters such as tapered fiber filters spliced into the erbium doped optical fiber. Filtration elements are commonly lossy components and thus, amplifier gain must be increased. However the use of tapered fiber filters reduces this effect due to their low background loss.
Preferably, the parameter of the optical fiber is the gain profile with respect to wavelength of the optical amplifier at that position along the optical fiber.
More preferably, the wavelength response of each of the optical filtration elements is selected to have a maximum loss at a wavelength corresponding to a maximum value of the gain profile of the erbium doped optical fiber at that position.
Preferably, the optical amplifier has a gain ripple of less than 3 dB over a wavelength range extending between 1530 nm and 1610 nm. The gain ripple may be less than 1 dB, and may even be as low a 0.1 dB to enable the amplifier to be used within a WDM system.
The use of the optical filtration elements arranged as described above enables a length of Er fiber to be used in the amplifier which can provide amplification of optical signals over a broad bandwidth. Furthermore, whereas in conventional broadband optical amplifiers there is a region of bandwidth (between the C and L bands) which is not usable, in the present invention a continuous band may be used.
Preferably, the length of the erbium doped fiber is greater than 20 m and more preferably, the length is between 25 m and 35 m.
The invention also provides a communications system having an optical fiber coupled between a first and second endstation, and a number of optical amplifiers of the invention connected to the optical fiber at positions along the fiber.
According to a second aspect of the present invention, there is provided an optical amplifier comprising an erbium doped optical fiber and a plurality of optical filtration elements provided at respective positions along the fiber, the fiber having a length of between 25 m and 35 m, and having at least five optical filtration elements along the length of the fiber, thereby giving the amplifier a gain ripple of less than 3 dB over a wavelength range extending between 1530 nm and 1590 nm.
Preferably, there are at least ten optical filtration elements.
The use of a large number of filters increases the efficiency of the amplifier. The filters have the effect of limiting the ASE and stopping its growth which would otherwise be exponential. This ensures that less power is wasted overall and increases the efficiency of the amplifier.